1. Field of the Invention
The present invention relates to seismic energy source arrays and, more particularly, to such source arrays which can be used to focus imparted energy towards displaced subsurface features.
2. Setting of the Invention
Vertical seismic profile (VSP) techniques are well-known to those skilled in the art and are very useful in acquiring information concerning the earth's subterranean features. Vertical seismic profiling generally comprises placing a plurality of receiver elements within a wellbore, and placing one or more source elements at the surface, either immediately adjacent the wellbore or spaced outwardly therefrom. A variation of VSP's is to have the receiver elements spaced outwardly on the surface and have the source element within the wellbore. While VSP's provides certain advantages over other seismic techniques, often times the resulting VSP seismic traces or data are barely acceptable in terms of imaging radially displaced subterranean features. Unacceptable seismic data often is acquired in areas of complex geology where energy reflections from secondary (nonimportant) features interfere with the energy reflections from the primary (important) feature to be investigated. Because of this problem, numerous seismic contractors and users of VSP data have been working on developing better VSP techniques to improve the quality of seismic energy traces obtained in these complex geological areas. Various VSP techniques dealing with enhancing the signals reflected from subsurface features are described in U.K. Patent Application GB No. 2029016A, and in the papers "Multi-Offset VSP Profiling" by Mons, OTC 5068, (1985) and "Use of Multiple Sources to Discriminate Between True and Virtual Reflectors in Imaging VSP Data," Noponen, S16.3, pages 770-772.
Further, a conventional source array acts as a single point of energy, because without ghost reflections the source array does not display any variation of amplitude with frequency and angle. Including ghost effects will tend to cause some amplitude variations. An amplitude response for a Prior Art source array is shown in FIG. 1A, with the signal amplitude reduction in decibels plotted as a function of radial angle from the wellbore and frequency. This Prior Art source array acts as a point source with no radial preference of response. The same affect can be shown by using Prior Art source arrays with vertical extent, with the source element within the wellbore.
FIG. 1B shows an amplitude spectra for 0.degree., 45.degree.and 70.degree. radials extracted from the data shown in FIG. 1A. Note that the energy response of the source array for the 0.degree. and 45.degree. radials are approximately equivalent, again confirming that the Prior Art source arrays having simply only either horizontal or vertical extent or very short distanced horizontal and vertical extent (such as 5' or less) act as point sources.
The problem with point sources is that in the case shown in FIG. 2 the energy passing along ray path B will be not much different from the energy passing along ray path A. Thus, in the processing, display and interpretation of the seismic data the energy from the two ray paths will tend to obscure the other. So, if one was trying to image the salt dome 14 one could not "steer"the energy coming from the same array towards the salt dome 14 and away from the boundary 12 so that the received energy along ray path A would be stronger than the received energy along ray path B.
There is a need for a VSP source array that can be used to enhance the reflections from the primary features and suppress the reflections from secondary features.